**Augustin Fresnel** did important work on optics where he was one of the founders of the wave theory of light.

- Jacques Fresnel was an architect who undertook major building works.
- We should note that Jacques and Augustine Fresnel were Jansenists; that is they were followers of Cornelius Otto Jansen (1585-1638) who led the Roman Catholic reform movement named after him.
- Augustin was brought up with strict Jansenist values in a stern atmosphere which would strongly influence him for the rest of his life.
- The French Revolution began with the storming of the Bastille on 14 July 1789 when Augustin was one year old.
- At age twelve Fresnel began his studies at the École Centrale in Caen.
- There was little doubt in Fresnel's mind at this time regarding his future career for he was firmly set on engineering.
- Fresnel was employed on a programme of road building which was designed to link this town with the rest of Vendée.
- Fresnel began working on this project in 1812 when he was based in Nyon but already he was undertaking scientific work in his spare time.
- One topic which fascinated Fresnel was that of light and he began to undertake experiments in the middle of 1814.
- Fresnel was so upset by this turn of events that he left his engineering job and offered to fight for the King against Napoleon.
- Of course this meant that Fresnel had put himself in a difficult position, and as a consequence he lost his engineering post and was put under police surveillance.
- Fresnel had few options left but to return to his home in Mathieu and this he did.
- Circumstances had conspired to give Fresnel the free time he needed to concentrate on his experiments with light.
- After Napoleon was defeated at Waterloo, Fresnel was reinstated into his old engineering appointment.
- By applying mathematical analysis to his work Fresnel removed many of the objections to the wave theory of light.
- Fresnel began by undertaking experiments with diffraction and made a breakthrough when he attached a piece of black paper to one edge of a diffracter and observed that then the bright bands within the shadow vanished.
- At this stage he had carried out fairly similar investigations that Thomas Young had carried out between 1797 and 1799 in Cambridge, but Fresnel next moved forward to a new understanding by developing a theory based on a new mathematical formulation.
- Fresnel published his first tentative results in July 1816 but asked that the readers of his article show patience while he worked out further consequences of the mathematics.
- It was a great chance for Fresnel to put his revolutionary work before the world and he was very confident of his theory since his mathematical deductions from the one simple hypothesis led to results which he had verified experimentally giving a highly accurate agreement between theory and experimental evidence.
- He completed his mathematical work just before the time for submission and this allowed him to calculate the intensity of light at every point behind the diffracter using what were later called Fresnel's integrals.
- In 1819 the committee to judge the Grand Prix of the Académie des Sciences, with Arago as chairman, and including Poisson, Biot and Laplace, met to consider Fresnel's submission.
- However Poisson was fascinated by the mathematical model which Fresnel proposed and succeeded in computing some of the integrals to find further consequences beyond those which Fresnel had deduced.
- This was a remarkable prediction, but Arago asked that Poisson's predictions based on Fresnel's mathematical model be tested.
- Indeed the bright spot was seen to be there exactly as Fresnel's theory predicted.
- Fresnel was awarded the Grand Prix and his work was a strong argument for a wave theory of light.
- Fresnel and Arago, now very confident that they could explain this effect with Fresnel's theory, undertook further work on polarisation and Fresnel discovered what was later called circularly polarised light.
- No hypothesis led to the experimental results obtained other than that light is a transverse wave and, in 1821, Fresnel published a paper in which he claimed with certainty that light is a transverse wave.
- Although Fresnel had made many converts to the wave theory of light, even from the most ardent of those previously believing in the corpuscular theory, his assertion that light is a transverse wave was a step too far for most.
- Even Arago dissented from this claim but Fresnel stunned his critics when he next showed that double refraction could be deduced from the transverse wave hypothesis.
- In 1823 Fresnel was elected to the Académie des Sciences.
- Fresnel died of tuberculosis in 1827 at the age of 39.
- Serious, intent, haunted by thoughts of an early grave, Fresnel bound himself closely to these ideals, shunning pleasures and amusements and working to the point of exhaustion.
- Despite the urgency of everything he attempted, Fresnel was always attentive to detail, systematic, and thorough.

Born 10 May 1788, Broglie, France. Died 14 July 1827, Ville-d'Avray, France.

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Physics

**O’Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive